Humans produce two flammable gases: hydrogen and methane. Flammable gases accumulate in an enclosed space and can ignite. Astronauts are humans who spend lots of time in enclosed space. The logic is irrefutable. So, what's the risk to farting astronauts?

Let's go retro for this story that we at io9 somehow missed when it was originally published in 1969. (No, I don't know how it slipped through the cracks either.) Thankfully, NCBI ROFL has unearthed this gem of a paper, which explored the potential buildup of hydrogen and methane in people on a "space diet," specifically the one used on the earlier Gemini missions. Here's what they found:

Breath and rectal gases were analyzed during the first and final weeks. Flatus gases varied widely within dietary groups but much more gas was generated with diet S than with F. In the first 12-hour collection, subjects fed S passed 3 to 209 ml (ATAP) of rectal H2 (avg 52) and 24 to 156 ml (avg 69) from the lungs (assuming normal pulmonary ventilation). With F, these values were 0 to 3 ml (avg 1) and 6 to 36 ml (avg 20). Subjects were calmer during the second test. Gas production was lower with S than initially; F values were unchanged. Methane differed idiosyncratically, presumably due to differences in flora. Computed from 12-hour values, maximum potential daily H2 and CH4 are per man: for S, 730 ml and 382 ml; for F, 80 and 222 ml. Volumes would be larger at reduced spacecraft and suit pressures.

Basically, the Gemini-style diet created way more flammable gas than the bland diet, which I guess means the Gemini astronauts went to truly herculean efforts to hold it in. Considering they were sharing a tiny capsule with just one other person, that maybe isn't that hard to believe. The scientific literature remains shamefully silent (Ed: but deadly?) on whether or not these findings were incorporated into NASA's work on future missions. We will assume the danger of farting astronauts has now passed, but I fear that may only be wishful thinking.

The study, incidentally, was the work of Edwin L. Murphy and Doris H. Calloway, who was a brilliant, ground-breaking nutrionist whose work far outshone my rather juvenile fixation on its fart-related aspects. (I can't find anything on Edwin L. Murphy, but I'll assume he was a brilliant scientist as well.)